Growing concerns about global warming and energy security demand the expansion of renewable energy sources as viable alternatives to fossil-fuel-based technologies, in conjunction with improved energy storage options. In many of the innovative approaches to address these challenges, the production of hydrogen in various (photo)-electrolysis systems plays a pivotal role. Today, electrolytically produced hydrogen comes mainly from the chloralkali industry and water electrolysis. In water-alkali electrolysers (WAEs), for example, the cathodic-half cell reaction is the hydrogen evolution reaction (HER), the electrochemical transformation of water to molecular hydrogen and hydroxyl ions (2H2O+2e−↔H2+2OH−). The mechanism of the HER is typically treated as a combination of three elementary steps: the Volmer step, water dissociation and formation of a reactive intermediate Had, (2H2O+M+2e−↔2M−Had+2OH−); followed by either the Heyrovsky step (H2O+Had-M+e−↔M+H2) or the Tafel recombination step (2M−Had↔2M+H2). Adsorbed hydrogen species Had formed at potentials negative of the Nernst reversible potential for the HER is also referred to as overpotentially deposited hydrogen (Hopd). The different states of adsorbed hydrogen can also be referred, based on thermodynamic guidelines, as Hupd—the strongly adsorbed state and Had/Hopd—a weakly adsorbed state. Although the reactions pathways are similar, due to the activated water dissociation step the HER activities for most catalysts in alkaline medium are usually ˜2-3 orders of magnitude lower than in acid solutions. The anodic-half cell reaction, the oxygen evolution reaction (OER), is a far more complex process, in which the hydroxyl ions generated at the cathode are consumed at the nio to produce oxygen and water molecules (4OH↔O2+2H2O+4e−). Given the harsh conditions associated with the OER, the choice of catalysts for electrolysis are typically noble metal oxides such as those of Ru, Ir and other forms of these. The poor conductivities and activities of the cheaper transition metal oxides such as that of 3d elements have limited their utilization in these systems. One way around it has been the use of high loadings of such materials.
Given that the supply of water is virtually inexhaustible, the hydrogen and oxygen production in WAEs can, in principle, be highly economical and almost limitless. In practice, however, large scale electrochemical production of hydrogen from water splitting is greatly constrained by two fundamental limitations: (1) the high overpotentials (defined as the difference between the reversible potential and the operating potential) of the HER and the OER in alkaline solutions, and (2) the lack of stability of electrode materials. The HER and the OER play key roles in a wide range of areas, including water and chlor-alkali electrolysis, metal deposition, corrosion, and fuel production from CO2 reduction. The HER is also an electrochemical reaction of fundamental scientific importance, since the basic laws of electrode kinetics, as well as many modern concepts in electrocatalysis, were developed and verified by examining the reaction mechanisms related to the charge-transfer-induced conversion of protons (acid solutions) and water (alkaline solutions) to molecular hydrogen.
It is not clear why the rate of the HER is ˜2 to 3 orders of magnitude lower at pH=13 than at pH=1, nor is it understood why the reaction is sensitive to the catalyst surface structure in alkaline media but largely insensitive in acids. A practical implication of the slow kinetics in alkaline solution is the lower energy efficiency for both water-alkali and chlor-alkali electrolyzers. For water-alkali electrolyzers, the high overpotentials for the oxygen evolution reaction (OER) at the anode also contribute significantly to overall energy losses. This has led to various approaches to identify catalysts for both OER and HER. However, rarely have these strategies for design of materials been based on molecular level understanding of the reaction pathways. In addition, the influence of non-covalent (Van der Waals type) interactions on the overall kinetics of the HER has been under explored, particularly in light of recent studies highlighting the impact of non-covalent interactions on the rates of many electrochemical reactions such as oxygen reduction reaction, CO and methanol oxidation reaction.
Design and synthesis of materials for efficient electrochemical transformation of water to molecular hydrogen and of hydroxyl ions to oxygen in alkaline environments is of paramount importance in reducing energy losses in water-alkali electrolysers. For decades, practical design of metal catalysts for the HER in acidic media has been based on the well-known concept of volcano plots. with rare exceptions, a classical volcano-shaped correlation is found from both experimental results as well as computational approaches; with metals that adsorb hydrogen neither too strongly nor too weakly (the Pt-group metals) occupying the top of volcano. While the metals that adsorb hydrogen too strongly (Ru, 3d-elements) are positioned on the descending part of the volcano, the IB group metals which exhibit a weak M−Had interaction on the ascending part. Similar plots also have been generated for the OER catalyst materials; for simple oxides, RuO2 and IrO2 exist at the apex of the volcano, with other transition metal oxides in both the ascending and descending portion of the curves. For more complex oxides, such as perovskites, similar positions exist with the metals in the ‘B’ site of the lattice determining the overall position in the volcano plot. One issue with the use of such volcano plots is the lack of clear information on active sites; for the theoretical calculation effort, ideal surfaces are used which seldom exist in reality. Similarly for experimentally derived, rarely are the materials well-defined, which results in several ambiguities due to the contributions from other factors such as defects, inhomogeneities etc. . . .
A great many materials have been tested for the HER and the OER in alkaline environments, including various combinations of metals, metal alloys, simple oxides such as RuO2/IrO2 (refs 15,16), and more complex materials such as combinations of 3d oxides, sulphides, phosphates and perovskites. Currently, various combinations of metals (pt, Pd, Ir, Ru, Ag, Ni), metal alloys (Ni—Co, Ni—Mn, Ni—Mo), metal oxides (RuO2), and Ni-sulfides/Ni-phosphides are used to catalyze the conversion of H2O to H2. Unfortunately, no current catalysts provide sufficient activity for hydrogen production (which is usually overcome with higher loading of these materials), thereby resulting in high overpotentials and energy losses. While most of the Pt group metals are good catalysts for the adsorption/recombination of the reactive hydrogen intermediates (Had), they are generally inefficient for the process of water dissociation. On the other hand, although metal oxides (and in some cases other compounds such as sulfides) are effective for cleaving the H—OH bond, they are highly ineffective in converting the resulting Had intermediates to H2. In addition, there are inherent issues with non-noble materials stemming from the decrease in activity during operation, arising from the formation of hydrides as well as the overall durability issues stemming from the dissolution of the catalyst materials during intermittent start-stop operations. Some of these issues have been overcome with alloying, in very high loadings of such catalyst materials (˜25-40 times the equivalent for Pt) in order to achieve the desirable activity. Similarly for the OER, given the harsh conditions, the stability of the materials is critical. Given the relatively low stability of most of these materials, the norm of using higher loading is common. Also, the limitation with development of new catalysts for the OER is the lack of clear fundamental knowledge required to design new catalysts.
Although these materials have shown interesting variations in catalytic behavior from one catalyst to the next, all of the currently used catalysts operate at high overpotentials. One of the major reasons for the slow progress in finding improved catalysts in WAEs is that the selection of these materials has been guided by a purely trial-and-error and/or a combinatorial approach, and no studies focusing on a systematic understanding of trends in the fundamental, atomic-scale catalytic properties of these reactions on well-characterized materials have been established. Current state of the art materials including oxides and metal catalysts are seldom cost effective, with noble metals having high materials cost and oxides having high performance cost.